1. Field of the Invention
The invention relates to a catalytic converter that has a casing in which a monolithic catalytic element is mounted by an intumescent vermiculite.
2. Description of the Related Art
Catalytic converters generally have a metallic casing encasing a monolithic catalytic element (herein referred to as a "monolith"). Because of the high temperatures encountered in the catalytic processes, the monolith usually is ceramic. The monolith typically is mounted in the casing by a material containing flakes of intumescent vermiculite ore which expand when heated to compensate for the greater thermal expansion of the metal of the casing as compared to the ceramic of the monolith.
Mountings containing flakes of intumescent vermiculite ore tend to exert high forces at temperatures at which a catalytic converter operates, which forces can be so high as to distort the metallic casing. Because a casing is more resistant to distortion when of circular cross section, some manufacturers avoid casings of elliptical cross section even though they would be helpful in the design of an automotive vehicle.
An intumescent mounting typically is in the form of a flexible mat which is wrapped around the monolith. The mat preferably incorporates inorganic fibrous material plus inorganic binder and/or organic binder which improve the integrity of the mat and thus make it easier to be wound onto a monolith and to stay in place.
Intumescent vermiculite mats are disclosed in co-assigned U.S. Pat. Nos. 3,916,057 (Hatch et al.) and U.K. Pat. 1,513,808 (issued Jun. 7, 1978). However, as is explained in co-assigned U.S. Pat. No. 4,305,992 (Langer et al.), the intumescent vermiculite of each has a region of negative expansion (contraction) between about 100.degree. and 400.degree. C. that can cause the monolith of a catalytic converter to become loose from the casing during the warm-up period of the converter. To guard against such loosening, the initial expansion temperature of the intumescent vermiculite should be as low as possible, and the intumescent vermiculite should not experience any negative expansion.
The Langer patent subjects flakes of vermiculite ore to an ammonium cationic exchange, both to reduce the degree of negative expansion and to reduce the temperature at which the intumescent mats begin to expand. U.S. Pat. No. 4,746,570 (Suzaki et al.) likewise subjects flakes of vermiculite ore to a cationic exchange plus a subsequent amine treatment in order to improve heat resistance and resiliency after repeated flexing at the high temperatures to which catalytic converters are subjected in use.
As explained in co-assigned U.S. Pat. No. 4,617,176 (Merry), pressure exerted against the monolith by intumescent vermiculite ore can be so high that, when combined with other factors, the monolith can experience "ring-off" cracks. In severe cases, the monolith is completely severed into two pieces. The Merry patent solves the ring-off problem by employing a flexible intumescent planar mat or sheet that is corrugated, i.e., it has a generally sinusoidal wave pattern along both its lengthwise edges, thus reducing forces exerted against the monolith. However, the corrugations of the intumescent sheet leave gaps that tend to allow heat to escape to the casing.
The Merry patent treats monolithic diesel particulate filters in the same manner as monolithic catalytic converters, and the term "catalytic converter" is meant to encompass monolithic diesel particulate filters. Like ordinary catalytic converters, many diesel monolithic particulate filters have a ceramic monolith mounted in a metallic casing, and the mounting can be a sheet of intumescent material.
Co-assigned U.S. Pat. No. 4,385,135 (Langer et al.) employs intumescent vermiculite for mounting a monolithic catalytic element and reduces "ring-off" cracking by substituting low density fillers in the vermiculite. Unfortunately, doing so has not provided the long-term durability desired in catalytic converters.
In addition to the above-discussed problems, flakes of intumescent vermiculite ore experience greater expansion when first heated to operating temperatures than in subsequent heating cycles. This thermal degradation results in gradually reduced holding forces at operating temperatures and may also result in gradual erosion of the vermiculite by exhaust gases.
The mounting material employed in many catalytic converters is a wire mesh that undergoes minimal expansion at increased temperatures, but a wire mesh affords less heat insulation than does an intumescent mat or sheet, so that it may be necessary to provide supplemental insulation at added cost. Furthermore, a wire mesh would allow exhaust gases to flow laterally between the casing and the monolith unless blocked in some way at additional cost. See, for example, U.S. Pat. No. 4,269,809 (Bailey et al.).